This invention concerns the measurement of intrauterine pressure and more specifically to a intrauterine catheter adapted for the measurement of intrauterine pressure during labor.
Fetal monitoring is a standard procedure for monitoring the condition of a fetus during childbirth. In most cases fetal heart rate and intrauterine pressure are measured and separately plotted on a strip-chart recorder. By examining these curves, the onset of certain distress conditions can be detected so that appropriate remedial action can be taken earlier than would otherwise be possible.
At present, to measure intrauterine pressure, an open-ended liquid filled catheter is inserted into the uterus so that the force of the intrauterine contractions can be transmitted through the uterine fluids and the liquid in the catheter to a pressure measuring device such as a strain gauge or the like.
In prior art applications of the type described, a cumbersome procedure is required to fill the catheter with liquid and then to couple the catheter to the strain gauge or the like to complete a liquid path or column from the uterus to the gauge.
Conventionally, the catheter is inserted through a relatively rigid guide tube which is curved to conform to the vaginal canal. The catheter is pushed through the guide tube until the uterine end of the catheter tube is located correctly within the uterus.
The catheter tube is coupled by means of an adaptor to a three-way stopcock, with two inlets of the stopcock being connected, respectively, to the lower fitting of the strain gauge and the syringe. The third inlet is "open to air". This enables the operator to open the system "to air" to check the electrical zero, i.e. "0" millimeters of mercury at any time during the recording to provide a means of calibrating the recorded data. This is accomplished by turning the "off" lever on the stopcock to the "open to air" inlet. The electrical zero can then be adjusted to "0" if necessary. The "off" lever is then turned towards the liquid filled syringe and liquid is injected into the system to purge any air from it and fill it again with liquid. The "off" lever is then turned towards the strain gauge closing the system.
During such an operation, if the "off" lever of the stopcock is improperly rotated, the force generated by the advancing syringe plunger may be applied directly against the transducer diaphragm, damaging it.
During use, presently available intrauterine catheters of the type described may have their openings "plugged" by clotted blood or other biologic debris present within the uterus. If this happens, pressure measurements are compromised and the catheter must be flushed to remove the debris and the system again bled as described above. If this is not successful, the catheter must be replaced.
Despite the use of a sterile liquid, because the liquid injection and air bleeding procedures take place in a non sterile environment, use of these standard catheters too frequently is accompanied by substantial risk of infection.
Other devices have been devised to avoid such a cumbersome procedure, including the invention of the co-applicants, Edward H. Hon, M.D., U.S. Pat. No. 4,136,681. In the device described therein, a catheter is disclosed for measuring the intrauterine pressure of a woman in labor. It comprises an elongated flexible tube which is inserted into the uterus through a curved guide tube adapted to be inserted through the vagina and cervix of a woman in labor. The catheter tube contains a sterile liquid and is closed at both ends by a seal which is capable of transmitting pressure from the liquid within the catheter to an external liquid. In a preferred embodiment, a limp membrane is used to couple the catheter tube to a strain gauge or the like for measuring pressure. The catheter end within the uterus includes a number of pinholes and a capillary material within the tube to prevent loss of the catheter liquid. Alternatively, and particularly where it is desired not to rupture the amniotic membrane, a second limp membrane may be used as the means for coupling between the catheter liquid and the uterus.
The use of the limp membrane at the uterine end of the catheter tube was found to be unacceptable due to the fact that during use it would frequently fold over itself, bend, or break due to the tension placed on it during insertion into the uterus.
Recently, disposable intrauterine catheters have been introduced, such as from Utah Medical, Inc., where a miniature transducer is placed on the catheter tip. (These are costly and represent a departure from the tried and true apparatus for measuring contraction.) Such devices have relatively large tips and are supported by relatively rigid wires which may cause damage to the fetus and uterine walls. Additionally, since the catheter is unprotected from its entrance to the vagina, until it is placed high in the uterus, the probability of transporting infectious material inside the uterus is high; the insertion of the large tip pushes bacteria high into the uterus where it is more likely to cause infection.
Also, in order to check the operations of such devices, once in place in the uterus, it is necessary to remove the catheter. This is highly undesirable, since it raises the probability of additional infection as well as being inconvenient and time-consuming.